Making Data Timelier and More Reliable with Lakehouse Technology

Making Data Timelier and More Reliable
with Lakehouse Technology
Matei Zaharia
Databricks and Stanford University

Talk Outline
§ Many problems with data analytics today stem from the complex data
architectures we use
§ New “Lakehouse” technologies can remove this complexity by enabling
fast data warehousing, streaming & ML directly on data lake storage

The biggest challenges with data today:
data quality and staleness

Data Analyst Survey
60% reported data quality as top challenge
86% of analysts had to use stale data, with
41% using data that is >2 months old
90% regularly had unreliable data sources

Data Scientist Survey
75%
51%
42%

Getting high-quality, timely data is hard…
but it’s partly a problem of our own making!

The Evolution of
Data Management

1980s: Data Warehouses
§ ETL data directly from operational
database systems
§ Purpose-built for SQL analytics & BI:
schemas, indexes, caching, etc
§ Powerful management features such as
ACID transactions and time travel
ETL
Operational Data
Data Warehouses
BI Reports

2010s: New Problems for Data Warehouses
§ Could not support rapidly growing
unstructured and semi-structured data:
time series, logs, images, documents, etc
§ High cost to store large datasets
§ No support for data science & ML
ETL
Operational Data
Data Warehouses
BI Reports

2010s: Data Lakes
§ Low-cost storage to hold all raw data
(e.g. Amazon S3, HDFS)
▪ $12/TB/month for S3 infrequent tier!
§ ETL jobs then load specific data into
warehouses, possibly for further ELT
§ Directly readable in ML libraries (e.g.
TensorFlow) due to open file format
BI Data
Science
Machine
Learning
Structured, Semi-Structured & Unstructured Data
Data Lake
Real-Time
Database
Reports
Data Warehouses Data
PreparationETL

Problems with Today’s Data Lakes
Cheap to store all the data, but system architecture is much more complex!
Data reliability suffers:
§ Multiple storage systems with different
semantics, SQL dialects, etc
§ Extra ETL steps that can go wrong
Timeliness suffers:
§ Extra ETL steps before data is available
in data warehouses
BI Data
Science
Machine
Learning
Data Lake
Real-Time
Database
Reports
Data Warehouses Data
PreparationETL

Problems with Today’s Data Lakes
Cheap to store all the data, but system architecture is much more complex!
Data reliability suffers:
§ Multiple storage systems with different
semantics, SQL dialects, etc
§ Extra ETL steps that can go wrong
Timeliness suffers:
§ Extra ETL steps before data is available
in data warehouses

Summary
At least some of the problems in modern data architectures are due to
unnecessary system complexity
§ We wanted low-cost storage for large historical data, but we
designed separate storage systems (data lakes) for that
§ Now we need to sync data across systems all the time!
What if we didn’t need to have all these different data systems?

Lakehouse Technology
New techniques to provide data warehousing features directly on
data lake storage
§ Retain existing open file formats (e.g. Apache Parquet, ORC)
§ Add management and performance features on top
(transactions, data versioning, indexes, etc)
§ Can also help eliminate other data systems, e.g. message queues
Key parts: metadata layers such as Delta Lake (from Databricks)
and Apache Iceberg (from Netflix) + new engine designs

Streaming
Analytics
BI Data
Science
Machine
Learning
Lakehouse Vision
Data lake storage for all data
Single platform for every use case
Management features
(transactions, versioning, etc)

Key Technologies Enabling Lakehouse
1. Metadata layers for data lakes: add transactions, versioning & more
2. New query engine designs: great SQL performance on data lake
storage systems and file formats
3. Optimized access for data science & ML

Metadata Layers for Data Lakes
§ A data lake is normally just a collection of files
§ Metadata layers keep track of which files are part of a table to enable
richer management features such as transactions
▪ Clients can then still access the underlying files at high speed
§ Implemented in multiple systems:
ACID
Keep metadata in the object store itself Keep metadata in a database

Problem: What if a query reads the table while the delete is running?
Example: Basic Data Lake
file1.parquet
file2.parquet
file3.parquet
“events” table Query: delete all events data about customer #17
file1b.parquet
file3b.parquet
rewrite
rewrite
+ delete file1.parquet
+ delete file3.parquet
Problem: What if the query doing the delete fails partway through?

Example with
file1.parquet
file2.parquet
file3.parquet
“events” table
_delta_log / v1.parquet
/ v2.parquet
Query: delete all events data about customer #17
file1b.parquet
file3b.parquet
rewrite
rewrite
track which files are part of
each version of the table
(e.g. v2 = file1, file2, file3)
_delta_log / v3.parquet
atomically add new log file
v3 = file1b, file2, file3b
Clients now always read a
consistent table version!
• If a client reads v2 of log, it sees
file1, file2, file3 (no delete)
• If a client reads v3 of log, it sees
file1b, file2, file3b (all deleted)
See our VLDB 2020 paper for details

Other Management Features with
§ Time travel to an old table version
§ Zero-copy CLONE by forking the log
§ DESCRIBE HISTORY
§ INSERT, UPSERT, DELETE & MERGE
SELECT * FROM my_table
TIMESTAMP AS OF “2020-05-01”
CREATE TABLE my_table_dev
SHALLOW CLONE my_table

Other Management Features with
§ Streaming I/O: treat a table as a
stream of changes to remove need
for message buses like Kafka
§ Schema enforcement & evolution
§ Expectations for data quality
CREATE TABLE orders (
product_id INTEGER NOT NULL,
quantity INTEGER CHECK(quantity > 0),
list_price DECIMAL CHECK(list_price > 0),
discount_price DECIMAL
CHECK(discount_price > 0 AND
discount_price <= list_price)
);
spark.readStream
.format("delta")
.table("events")

Adoption
§ Used by thousands of companies to process exabytes of data/day
§ Grew from zero to ~50% of the Databricks workload in 3 years
§ Largest deployments: exabyte tables and 1000s of users

Available Connectors
Store
data in
Ingest
from
Query
from

The Challenge
§ Most data warehouses have full control over the data storage system
and query engine, so they design them together
§ The key idea in a Lakehouse is to store data in open storage formats
(e.g. Parquet) for direct access from many systems
§ How can we get great performance with these standard, open formats?

Enabling Lakehouse Performance
Even with a fixed, directly-accessible storage format, four optimizations
can enable great SQL performance:
§ Caching hot data, possibly in a different format
§ Auxiliary data structures like statistics and indexes
§ Data layout optimizations to minimize I/O
§ Vectorized execution engines for modern CPUs
New query engines such as Databricks Delta Engine use these ideas

Optimization 1: Caching
§ Most query workloads have concentrated accesses on “hot” data
▪ Data warehouses use SSD and memory caches to improve performance
§ The same techniques work in a Lakehouse if we have a metadata layer
such as Delta Lake to correctly maintain the cache
▪ Caches can even hold data in a faster format (e.g. decompressed)
§ Example: SSD cache in
Databricks Delta Engine
0 20 40 60 80
Parquet onS3
Parquet onSSD
Delta Engine cache
Values read per second per core (millions)

Optimization 2: Auxiliary Data Structures
§ Even if the base data is in Parquet, we can build many other data
structures to speed up queries and maintain them transactionally
▪ Inspired by the literature on databases for “raw” data formats
§ Example: min/max statistics on Parquet files for data skipping
file1.parquet
file2.parquet
file3.parquet
year: min 2018, max 2019
uid: min 12000, max 23000
uid: min 12000, max 14000
uid: min 23000, max 25000
Query: SELECT * FROM events
WHERE year=2020 AND uid=24000
updated transactionally
with Delta table log

Optimization 2: Auxiliary Data Structures
§ Even if the base data is in Parquet, we can build many other data
structures to speed up queries and maintain them transactionally
▪ Inspired by the literature on databases for “raw” data formats
§ Example: indexes over Parquet files
file1.parquet
file2.parquet
file3.parquet
Query: SELECT * FROM events
WHERE type = “DELETE_ACCOUNT”
tree
index

Optimization 3: Data Layout
§ Query execution time primarily depends on amount of data accessed
§ Even with a fixed storage format such as Parquet, we can optimize the
data layout within tables to reduce execution time
§ Example: sorting a table for fast access
file1.parquet
file2.parquet
file3.parquet
file4.parquet
uid = 0…1000
uid = 1001…2000
uid = 2001…3000
uid = 3001…4000

Optimization 3: Data Layout
§ Query execution time primarily depends on amount of data accessed
§ Even with a fixed storage format such as Parquet, we can optimize the
data layout within tables to reduce execution time
§ Example: Z-ordering for multi-dimensional access
dimension 1
dimension2
99%
67%
0%
60%
0%
47%
0%
44%
0%
20%
40%
60%
80%
100%
Sort by col1 Z-Order by col1-4
DataFilesSkipped
Filter on col1 Filter on col2
Filter on col3 Filter on col4

Optimization 4: Vectorized Execution
§ Modern data warehouses optimize CPU time by using vector (SIMD)
instructions on modern CPUs, e.g., AVX512
§ Many of these optimizations can also be applied over Parquet
§ Databricks Delta Engine: ~10x faster
than Java-based engines
3220
25544
35861
0
10000
20000
30000
40000
Delta
Engine
Apache
Spark 3.0
Presto 230
TPC-DS Benchmark Time (s)

Putting These Optimizations Together
§ Given that (1) most reads are from a cache, (2) I/O cost is the key factor
for non-cached data, and (3) CPU time can be optimized via SIMD…
§ Lakehouse engines can offer similar performance to DWs!
0
10000
20000
30000
40000
DW1 DW2 DW3 Delta
Engine
TPC-DS 30TB Benchmark Time (s)

ML over a Data Warehouse is Painful
§ Unlike SQL workloads, ML workloads need to process large amounts of
data with non-SQL code (e.g. TensorFlow, XGBoost, etc)
§ SQL over JDBC/ODBC interface is too slow for this at scale
§ Export data to a data lake? → adds a third ETL step and more staleness!
§ Maintain production datasets in both DW & lake? → even more complex

ML over a Lakehouse
§ Direct access to data files without overloading the SQL frontend
(e.g., just run a GPU cluster to do deep learning on S3 data)
▪ ML frameworks already support reading Parquet!
§ New declarative APIs for ML data prep enable further optimization

Example: Spark’s Declarative DataFrame API
Users write DataFrame code in Python, R or Java
users = spark.table(“users”)
buyers = users[users.kind == “buyer”]
train_set = buyers[“start_date”, “zip”, “quantity”]
.fillna(0)

Example: Spark’s Declarative DataFrame API
Users write DataFrame code in Python, R or Java
...
model.fit(train_set)
Lazily evaluated query plan
Optimized execution using
cache, statistics, index, etc
users
SELECT(kind = “buyer”)
PROJECT(start_date, zip, …)
PROJECT(NULL → 0)users = spark.table(“users”)
buyers = users[users.kind == “buyer”]
train_set = buyers[“start_date”, “zip”, “quantity”]
.fillna(0)

ML over Lakehouse: Management Features
Lakehouse systems’ management features also make ML easier!
§ Use time travel for data versioning and reproducible experiments
§ Use transactions to reliably update tables
§ Always access the latest data from streaming I/O
Example: organizations using Delta Lake as an ML “feature store”

Summary
Lakehouse systems combine the benefits of data warehouses & lakes
§ Management features via metadata
layers (transactions, CLONE, etc)
§ Performance via new query engines
§ Direct access via open file formats
§ Low cost equal to cloud storage
Streaming
Analytics
BI Data
Science
Machine
Learning
Result: simplify data architectures to
improve both reliability & timeliness

Before and After Lakehouse
Typical Architecture with Many Data Systems
ETL Job ETL Job
ETL Job
Delta Lake
Table 1
Delta Lake
Table 2
Delta Lake
Table 3
Streaming
Analytics
Data
Scientists
BI Users
Cloud Object Store
input
Cloud Object Store
ETL Job ETL Job
ETL Job
Message
Queue
Parquet
Table 1
Parquet
Table 2
Parquet
Table 3
Data
Warehouse
Data
Warehouse
Streaming
Analytics
Data
Scientists
BI Users
input
Lakehouse Architecture: All Data in Object Store
Fewer copies of the data, fewer ETL
steps, no divergence & faster results!

Learn More
Download and learn Delta Lake at delta.io
View free content from our conferences at spark-summit.org:

Making Data Timelier and More Reliable with Lakehouse Technology

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